The present invention relates generally to an emission spectroscopic processing apparatus for spectrally separating radiation emitted from a plasma or the like into component spectra, converting the component spectra having respective wavelengths into electric signals by means of associated light receiving elements, respectively, and obtaining a desired detection output by processing the signals. Further, the present invention relates to a plasma processing method using the emission spectroscopic processing apparatus.
The emission spectroscopic processing apparatus for spectrally separating radiation emitted from a plasma or the like into component spectra, converting the component spectra having respective wavelengths into electric signals by means of light receiving elements or devices and obtaining a desired detection output by processing the signals has been known heretofore. By way of example, a process monitoring apparatus adopting a main component analysis process mentioned below is disclosed, for example, in European Patent No. 1089146.
More specifically, electromagnetic radiation emitted from a plasma camber is inputted into a process monitoring apparatus which is composed of a spectrometer and a processor through the medium of an optical fiber or the like. The spectrometer mentioned above is designed to spatially split or separate the electromagnetic radiation of plasma on the basis of the wavelengths by using, for example, a prism or a diffraction grating. Subsequently, a plurality of spatially separated spectra of respective wavelengths are detected by means of e.g. a CCD (Charge Coupled Device) array of 2048 channels, whereby a detection signal, i.e., OES (Optical Emission Spectroscopic) signal is generated. The OES signal is then digitized through e.g. an analogue-to-digital (A/D converter) to be outputted to a processor for undergoing further processings. In this manner, the electromagnetic radiation emitted from a plasma is measured by the spectrometer and supplied to the processor in the form of the OES signal of 2048 channels.
Main or major component analysis process of a specific or desired type to be executed by the processor is selected by means of a remote computer system, fabrication equipment or the like. In place of the spectrometer, there may be employed a diffraction grating, prism, optical filter or other type of wavelength selecting device(s) in combination with a plurality of detectors (e.g. photodiodes, photo multipliers or the like) to thereby supply the information concerning a plurality of electromagnetic radiation wavelengths to the processor. In this conjunction, it is to be added that the processor is coupled to a plasma etching controller by way of a control bus.
In practice, the CCD array is used in many applications as the convenient means for making available radiation amplitude signals corresponding to the wavelengths of the component spectra. However, in the CCD array which is constituted by a large number of integrated light receiving elements, such a problem is encountered that when the light receiving elements each of small capacity are employed in an effort to increase the sensitivity, then noise also increases, whereas when the light receiving elements of large capacity are employed with a view to suppressing the noise, the sensitivity of the CCD array will become lowered. By way of example, in the case of a CCD array of a relatively high sensitivity (e.g. 2048-pixel CCD linear sensor xe2x80x9cILX511xe2x80x9d commercially availably from Sony Co. Ltd), the signal-to-noise ratio (S/N ratio) is on the order of 250 in the state where a quantity of light of saturation level is received, and the S/N ratio decreases in proportion to the one-second power (xc2xd) of the received light quantity as it deceases. This problem is not inherent to the CCD array but generally common to photosensor devices each composed of a large number of integrated light receiving elements.
In the ordinary image sensor, a mean value of the received light quantity distribution over the whole image or a peak value thereof is measured for the purpose of effectuating a gain adjustment for changing the amplification factor for the output signal of the CCD array or the charge storing time thereof on the basis of the measured value in order to cope with changes or variations in the quantity of incident light or radiation. In this conjunction, reference may be made to, for example, Japanese Patent Application Laid-Open Publication No. 324297/2000 and USP 2001/0016053A1.
On the other hand, in the plasma processing apparatus, the incident radiation quantity may change remarkably (ca. ten times or more) due to aged contamination of a process chamber. For coping with such change of the incident radiation quantity, it is not preferred to change the charge storing time of the CCD array because then the operation timing of the whole system will have to be changed remarkably. Further, in the spectrum of plasma emission produced in the plasma processing apparatus, there are coexistent mixedly a plurality of high luminance portions exhibiting steep peaks and low luminance portions changing relatively gently as a function of the wavelength (see e.g. U.S. Pat. No. 6,261,470B1, FIG. 17A or European Patent No. 1089146, FIG. 3C). In the applications where the emission spectra are detected by using the CCD array, setting of the amplification factor for the output signal of the CCD array so that no saturation can occur at the steep peaks will inevitably be accompanied with remarkable degradation of the S/N ratio for the low luminance portion of the radiation. On the contrary, when the amplification factor is set in conformance with the low luminance portion, saturation will easily occur in the peak portions.
In a semiconductor fabrication apparatus, the time-dependent changes of emission spectra emitted from a process chamber of the apparatus (i.e., change of the emission spectra in the course of times lapse) indicate changes in the contents of processing or treatment being carried out within the chamber. In recent years, it has been practiced to estimate the process situation or statuses within the process chamber on the basis of extremely small or minute changes of the emission spectra. However, in the case where the CCD array or the like device is used as the means for detecting the emission spectra, the signal which can be made available is very poor in respect to the S/N ratio as mentioned previously. Such being the circumstances, addition of the signal of a same wavelength is repeated a number of times in an effort to eliminate the noise components. However, with this method, it is necessary to repetitively perform the addition of a sample one hundred times or more if the signal-to-noise ratio is to be increased by one order of magnitude. Such processing will ordinarily require several to several ten seconds, which in turn renders it relatively difficult to detect the minute change (change of less than 10% or so) of high rate or speed on the order of one second or 0.5 second or lower in terms of temporal duration. In particular, in a low luminance portion which changes relatively gently as a function of the wavelength in the emission spectra such as of plasma, great difficulty will be encountered in detecting the minute change of high rate on the order of one second or less with satisfactory reproducibility.
In the light of the state of the art described above, it is an object of the present invention to provide an emission spectroscopic processing apparatus which is capable of detecting minute changes in emission spectra of high rate or speed on the order of one second or less with enhanced or improved reproducibility.
Another object of the present invention is to provide a plasma processing method which is carried out by using the emission spectroscopic processing apparatus mentioned above.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to an aspect of the present invention an emission spectroscopic processing apparatus which includes a spectroscope for spectrally separating input light emitted from a process unit into component spectra, a light receiving unit including a series of light receiving elements for detecting light quantities of the component spectra on a wavelength-by-wavelength basis, a first signal hold circuit for holding sequentially each of detection signals outputted from subsets of adjacent light receiving elements contained in said series of light receiving elements for a first period, respectively, an adder unit for adding together the detection signals of adjacent light receiving elements of the light receiving unit inclusive of the held detection signals of the subset of the adjacent light receiving elements, a second signal hold unit for holding sequentially sum outputs of the adder unit, and a signal processing unit for determining a state of the process unit on the basis of the output of the second signal hold unit.
In a preferred mode for carrying out the invention, the emission spectroscopic processing apparatus includes a light receiving unit comprised of a series of light receiving elements for detecting light quantities of the component spectra on a wavelength-by-wavelength basis, an adder unit for adding together the detection signals outputted from light receiving elements which correspond to a set of emission wavelengths intrinsic to preset light emission materials, respectively, a third signal hold unit for holding sequentially sum outputs of the adder unit, and a signal process unit for determining a state of the processing unit on the basis of the output of the third signal hold unit.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.